CO 2 recovery from CPU vent of CFB oxyfuel plants by Ca-looping process

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1 CO 2 recovery from vent of CFB oxyfuel plants by Ca-looping process M.C. Romano 5 th IEAGHG Network Meeting - High Temperature Solid Looping Cycles 2-3 September 2013, Cambridge, UK

2 Motivations of the study 2 Oxyfuel power plants produce concentrated CO 2 streams with some non-condensable gases (5-25% on dry basis) from: O 2 purity from ASU < 100% Infiltration air in equipment below ambient pressure (mainly boiler and filters) Oxidant excess for complete combustion Nitrogen in fuel CO 2 purification unit () is likely to be needed to comply with purity specifications of injection site and CO 2 transport pipeline final CO 2 purity should be from 95% to >99% Typically, auto-refrigerated processes based on low temperature flash separation are proposed some CO 2 is inevitably vented with the non-condensable gases CO 2 capture efficiency drops from potentially 100% to 85-95% CO 2 emission g/kwh (1/3-1/5 of a NG-fired combined cycle)

3 Motivations of the study 3 Question: is it economical to recover this vented CO 2, which is available in a clean stream at relatively high concentrations (30-45%vol.)? Probably yes! Options proposed up to now in the literature: Praxair: Vacuum pressure swing adsorption (VPSA) Air Products and Air Liquide: polymeric membranes Why is Ca-looping a good alternative: >90% of the CO 2 can be recovered from the vent overall CO 2 capture efficiency 99% In case of CFB boiler, CaL purge can be reused in the main boiler for sulfur capture no additional solid material to be imported or displaced No significant thermodynamic penalty is expected: coal burned in the calciner is converted with approximately the same efficiency of the coal burned in the main boiler (the overall amount O 2 to be produced does not change)

4 Plant layout 4 CaL section 21 CARBONATOR infiltration air 24 3 CALCINER coal limestone CFB boiler & power plant DCC ESP waste 29 FF N2 ASU expanders Non-condesable off-gas Drier CO2 to storage 12

5 Main assumptions 5 24 infiltration air 3 DCC 9 21 CARBONATOR CALCINER limestone coal ESP waste 29 FF CFB BOILER Operating temperature, C 850 Non-condesable off-gas Oxygen concentration in flue gas, %vol. 2.5 Drier expanders Oxygen preheating temperature, C 200 Air in-leakages, %wt. of flue gas 30 1 Recycle gas temperature, C Ca/S CO2 to storage 12 Gas temperature at DCC inlet, C 175 Desulphurization efficiency, % In the CFB boiler, no recarbonation of CaO contained in CaL purge initially assumed: conservative assumption N2 ASU

6 Main assumptions 6 STEAM CYCLE Boiler feedwater temperature, C Live steam temperature SH/RH, C 600/620 infiltration air 24 Live steam pressure SH/RH, C 3 270/60 CARBONATOR CALCINER Condensing pressure, bar coal limestone DCC STEAM TURBINE 23 Number of HP/IP/LP 20 parallel 26 flows 1/2/4 ESP Isentropic efficiency, % FF Last stage peripheral velocity at mean diameter, m/s Exhaust steam velocity, m/s expanders Non-condesable off-gas Drier calculated Last stage turbine blade height, m waste N2 ASU CO2 to storage 12

7 Main assumptions 7 24 infiltration air 3 DCC 9 21 CARBONATOR CALCINER limestone coal ESP waste 29 FF N2 ASU expanders Non-condesable off-gas 10 Drier Number of intercooled compressors 5 Temperature at knockout drum, C -54 Minimum ΔT in main heat exchanger, C 2 Final CO 2 purity, %mol 97 Final CO 2 pressure, bar CO2 to storage 12

8 Main assumptions 8 FF expanders Non-condesable off-gas 10 CARBONATOR 19 Drier CALCINER limestone infiltration air coal CaL SECTION Carbonator: Operating temperature, C CO 2 capture efficiency, % Inventory, kg/m 2 ESP 4 Gas superficial velocity, m/s Riser height, m 5 Temp. of the CO 2 -lean gas to the 6 stack, C Calciner Operating temperature, C Calcination efficiency, % 39 Oxygen in oxidant stream, %vol. Oxygen concentration in flue gas, %vol DCC 7 waste N2 8 ASU Sorbent make-up F 0 determined by the Ca/S ratio in the CFB boiler Sorbent circulation rate F R,Ca calculated to obtain the target CO 2 capture CO2 to storage 12

9 Simulation tools 9 GS code ( Modular structure: very complex schemes can be reproduced by assembling basic modules Efficiency of turbomachineries evaluated by built-in correlations accounting for operating conditions and the machine size Stage-by-stage calculation of steam and gas turbines Calculation of chemical equilibrium based on Gibbs free energy Thermodynamic properties of gases NASA polynomials Thermodynamic properties of water/steam IAPWS-IF97 Aspen Plus: CO 2 compression and purification Matlab: Carbonator model (M.C. Romano, 2012: Modeling the carbonator of a Ca-looping process for CO 2 capture from power plant flue gas; Chem Eng Sci, 69, )

10 SPECCA index 10 Specific primary energy consumption for CO 2 avoided: Two options for the reference plant used: Ref = reference plant without CO 2 capture SPECCA Ref = reference oxyfuel plant without CaL SPECCA-CaL (penalty associated to the addition of the CaL unit)

11 Results 11 South African coal: 0.52%wt. Sulfur; MJ LHV /kg Air boiler oxy-cfb oxy-cfb-cal CaL- - no yes Coal thermal input, MWLHV Electric power balance, MW Steam turbine Steam cycles auxiliaries Fans ASU CO 2 compression Other auxiliaries Net electric plant output, MW Gross electric efficiency, %LHV Net electric efficiency, %LHV Carbon capture ratio, % CO 2 specific emission, g/kwh CO 2 avoided, % SPECCA, MJ/kgCO SPECCA-CaL, MJ/kgCO

12 Results & CaL 12 CO 2 in impure stream to, %mol. dry 87.3 CO 2 content in the non-condensable stream, %mol Fraction of the inlet CO 2 in the non-condensable stream, % 7.1 CaL PROCESS Specific limestone make-up, F 0 /F CO Specific recycle flow rate F R,Ca /F CO2 7.5 Carbonator inventory, t 12.1 Carbonator diameter, m 3.20 Calciner diameter, m 5.19 Heat input to the calciner, MW LHV Fraction of the total plant heat input to the calciner, % 9.4

13 13 Results effect of sulfur in coal and CaO recarbonation Illinois #6 coal: 3.41%wt. Sulfur F 0 /F CO2 =0.49; F R,Ca /F CO2 =3.6 Air boiler oxy-cfb Oxy-CFB-CaL CaL- - no yes CaO recarbonation - - no yes Coal thermal input, MWLHV Electric power balance, MW Steam turbine Steam cycles auxiliaries Fans ASU CO 2 compression Other auxiliaries Net electric plant output, MW Gross electric efficiency, %LHV Net electric efficiency, %LHV Carbon capture ratio, % CO 2 specific emission, g/kwh CO 2 avoided, % SPECCA, MJ/kgCO SPECCA-CaL, MJ/kgCO

14 Conclusions 14 CaL can really reduce the emission from oxyfuel CFB plants and compete with other proposed processes, at least from the performance side. An economic analysis would be needed to fully understand the potential of this process and to fully optimize it. Also the optimal process parameters might change if CO 2 recovery is carried out. A better understanding of the behavior of CaO in the oxyfuel boiler would be important: carbonation at C, high CO 2 concentration, low Ca/CO 2 and Ca/S sulfation

15 15 Thank you The complete discussion on: M.C. Romano: Ultra-high CO 2 capture efficiency in CFB oxyfuel power plants by calcium looping process for CO 2 recovery from purification units vent gas; Int J Greenh Gas Control 18, 57-67, 2013.